A study was undertaken to determine whether the positive correlation between Chlamydia trachomatis antibodies and tubal infertility, noted by workers in other countries, also applied to infertile women in Northern Ireland. Ninety-one infertile women and 106 fertile controls were tested for current cervical infection with C. trachomatis and for evidence of past chlamydial infection. The incidence of C. trachomatis infection of the cervix was 5.8% in the infertile group and 2.8% in the control group. The prevalence of C. trachomatis antibody was 22% in the infertile group and 18.9% in the control group. Previous termination of pregnancy, history of sexually transmitted disease and number of sexual partners were identified as risk factors for seropositivity and tubal disease. We concluded that it would be of value to screen women attending the infertility clinic for C. trachomatis infection of the cervix, and that testing these patients for chlamydia antibodies may also be useful in planning further investigation.